1. Field of the Invention
The present invention relates to a method for manufacturing semiconductor devices and particularly to a method for surface treatment and forming low contact resistance wirings.
Recently, with further improvement in high packing density of semiconductor devices, multilayer wirings are often used and moreover since the contacts of wirings are also microminiaturized, contact resistance increases.
Meanwhile, depth of p-n junction formed becomes shallow and thereby leakage current resulting from damage at the p-n junction in the contact area increases.
When contact resistance becomes larger up to a degree as cannot be neglected for channel resistance the leakage current, for channel current, since these factors have a large influence on operations of the device, reduction of contact resistance and leakage current is essential.
Moreover, flatness of the substrate is also essential for improving reliability of wirings such as stability of wirings for heat cycles.
2. Description of the related Art
In the case of forming contact layers of an aluminum system or refractory metal on a silicon substrate by the sputtering method or CVD method, it is known that the cleanliness of the substrate surface influences the contact resistance. For instance, when titanium(Ti) is formed on a Si substrate by the sputtering method, formation of silicide (TiSi.sub.x) generated through the reaction of Ti and Si becomes non-uniform due to contamination of the Si surface (natural oxide, organic substances, etc). Contact resistance therefor becomes high. Moreover, when a tungsten (W) layer is grown by the CVD method, since nucleation is not smoothly formed on a natural oxide, the layer does not grow or the layer is grown with irregular corrosion.
Therefore, it is essential to keep the surface clean to form the contacts in order to attain a low contact resistance. As the surface treatment of the lower base layer, the wet etching method and dry etching method such as plasma etching method and hydrogen reduction method have been conducted. However, using a wet or dry etching method, side products may easily be generated and using the plasma etching method, damage occurs on the surfaces. Moreover, int he reduction by hydrogen, thermal damage occurs. The diffused layer is extended or the aluminum wirings are easily melted due to high temperature (800.degree. C.) and as a result, the characteristics of the device may be lowered. As explained above, the best surface processing method has not yet been found.
In order to realize low contact resistance, refractory metal (for example, tungsten W or titanium Ti), silicide thereof (for example, WSi.sub.x, TiSi.sub.x) or nitride thereof (for example, WN. TiN) are used in place of aluminum (Al) which has been used most widely as the conventional wiring material. These materials simultaneously play the role of a barrier metal for interfering with chemical reactions between the substrate and the aluminum wirings. Even when the contact includes shallow junctions, a spike generated by the chemical reactions between the substrate materials and the wiring materials does not damage the shallow junctions. Therefore, this barrier metal effectively prevents leakage current caused by damage on the junction areas.
With regard to the contact metal layer, the method for sputtering the Al related material such as aluminum (Al), AlSi and AlCu has been used. However, in this case, there have been the disadvantages that AlSi allows an increase of the contact resistance by segregation of si and that defective wirings occur due to low migration resistance (electro migration or stress migration) of Al and Al alloy.
This migration problem has been solved by using the CVD method using tungsten W as the contact metal layer or wirings. Coverage of Al is improved by the burying method and defective wirings do not occur due to high migration resistance of wirings by tungsten W. However, this CVD method still provides a disadvantage that Si or fluorine F, which is an element of the reactant gas forming element, is easily incorporated in the tungsten W layer with a low temperature reducing reaction using silane (SiH.sub.4), etc. As a result, contact resistance becomes larger and the coverage rate becomes smaller. On the other hand, the CVD of tungsten W using the reducing reaction of hydrogen (H.sub.2) is characterized by providing a large coverage rate but is also accompanied by the disadvantage that corrosion of Si is distinctive and leak current is generated due to high growth temperature and generation of corrosive gas.
The tungsten W layer obtained by the CVD method provides a problems such as peeling of the layer due to having a bad adhesive property on the SiO.sub.2 layer. Therefore, an adhesive layer which also works as a barrier layer of contact such as TiN is first formed by the sputtering method and a tungsten W layer is formed thereon also by the CVD method. However, this adhesive layer has bad coverage at the stepped portions due to use of the sputtering method and causes abnormal growth, zero growth or peeling of the tungsten layer at the contact regions.
Accordingly, is has long been expected that the optimum method for cleaning the surface of substrate had been found and a lower contact resistance can be obtained by forming wiring materials on the surface of base substrate.
Moreover, the most desirable method is to sequentially form the barrier metal layer and wiring materials within a series of processings including the surface treatment of base substrate.